Silicon is one of the most promising semiconductor materials for spin-based information processing devices. Its advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of a (28)Si form with no magnetic nuclei overcomes a primary source of spin decoherence in many other materials. Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than that achieved in isolated high-vacuum systems such as trapped ions. Here we examine electron spin coherence of donors in pure (28)Si material (residual (29)Si concentration <50 ppm) with donor densities of 10(14)-10(15) cm(-3). We elucidate three mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime T(2) up to 2 s. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by ~200 nm. A magnetic field gradient suppresses such interactions, producing an extrapolated electron spin T(2) of 10 s at 1.8 K. These coherence lifetimes are without peer in the solid state and comparable to high-vacuum qubits, making electron spins of donors in silicon ideal components of quantum computers, or quantum memories for systems such as superconducting qubits.

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Long-distance entanglement distribution is a vital capability for quantum technologies. An outstanding practical milestone towards this aim is the identification of a suitable matter-photon interface that possesses, simultaneously, long coherence lifetimes and efficient telecommunication-band optical access. In this work we report upon the T center, a silicon defect with long-lived spins and spin-selective bound exciton optical transitions at 1326 nm in the telecommunications O-band. In this first study of T centers in 28 Si, we present the temperature dependence of the zero-phonon line, report ensemble zero-phonon linewidths as narrow as 33(2) MHz, and elucidate the excited state spectrum of the bound exciton. Magnetophotoluminescence, in conjunction with magnetic resonance, is used to observe twelve distinct orientational subsets of the T center, which are independently addressable due to the anisotropic g factor of the bound exciton's hole spin. Here we show that the T center in 28 Si offers electron and nuclear spin lifetimes beyond a millisecond and second, respectively, as well as optical lifetimes of 0.94(1) μs and a Debye-Waller factor of 0.23(1). This work represents a significant step towards coherent photonic interconnects between longlived silicon spins, spin-entangled telecom single-photon emitters, and spin-dependent silicon-integrated photonic nonlinearities for future global quantum technologies.

Chalcogen donors in silicon enable a scalable photonic cavity quantum electrodynamics solution for universal quantum computing.

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